Inkjet printers eject drops of ink through an array of nozzles to effect printing on a media substrate. The nozzles are typically formed on a silicon wafer substrate using semiconductor fabrication techniques. Each nozzle is a MEMS (micro electromechanical systems) device driven by associated drive circuitry formed on the same silicon wafer substrate. The MEMS nozzle devices and associated drive circuitry formed on a single nozzle is commonly referred to as a printhead integrated circuit (IC).
Traditional inkjet printers use scanning inkjet printheads. These have a single printhead IC that traverses back and forth across the width of a page as the printer indexes the page along. The Applicant has developed a range of pagewidth printheads. These printheads use a series of printhead IC's mounted end to end to provide an array of nozzles that extends the entire width of the page. Instead of scanning back and forth, the printhead remains stationary in the printer as the page is fed past. This allows much higher print speeds but is more complicated in terms of controlling the operation of a much larger array of nozzles.
Thermal inkjet printheads, or Bubblejet™ printheads as they are commonly known (‘Bubblejet’ is the trademark of Canon), eject ink by generating a vapor bubble. Each nozzle has a corresponding a heater element that superheats the ink to vaporize surrounding ink to generate the bubble that ejects a drop of ink onto the paper. Heat dissipates into the ink as the heater temperature rises to the bubble nucleation temperature. Because of this, the temperature of the ink in a nozzle will depend on how frequently it is being fired at that stage of the print job. A pagewidth printhead has a large array of nozzles and at any given time during the print job, a portion of the nozzles will not be ejecting ink. The ink in these non-ejecting nozzles will be dissipating heat and cooling down. In contrast, nozzles that are firing a series of drops will contain ink that is maintained at a higher temperature. Unfortunately, the ink viscosity is temperature dependent and therefore the ink viscosity will also vary depending on the temperature of the nozzle. The viscosity of the ink affects the drop ejection characteristics of the nozzle. For example, ink with lower viscosity forms a larger ejected drop from a given actuation pulse than ink with a higher viscosity. This can lead to visible artifacts in the printed page.